Bottom Line:
More surprisingly chimpanzees were crop-raiding during the night.While nocturnal activities of chimpanzees have been reported during full moon periods, this is the first record of frequent and repeated nocturnal activities after twilight, in darkness.Habitat destruction may have promoted behavioural adjustments such as nocturnal exploitation of open croplands.

ABSTRACTIn a rapidly changing landscape highly impacted by anthropogenic activities, the great apes are facing new challenges to coexist with humans. For chimpanzee communities inhabiting encroached territories, not bordered by rival conspecifics but by human agricultural fields, such boundaries are risky areas. To investigate the hypothesis that they use specific strategies for incursions out of the forest into maize fields to prevent the risk of detection by humans guarding their field, we carried out video recordings of chimpanzees at the edge of the forest bordered by a maize plantation in Kibale National Park, Uganda. Contrary to our expectations, large parties are engaged in crop-raids, including vulnerable individuals such as females with clinging infants. More surprisingly chimpanzees were crop-raiding during the night. They also stayed longer in the maize field and presented few signs of vigilance and anxiety during these nocturnal crop-raids. While nocturnal activities of chimpanzees have been reported during full moon periods, this is the first record of frequent and repeated nocturnal activities after twilight, in darkness. Habitat destruction may have promoted behavioural adjustments such as nocturnal exploitation of open croplands.

pone-0109925-g004: Frequency of signs of anxiety and vigilance in chimpanzees during day and night crop-raiding (occurrence of each behaviour per minute of video record).

Mentions:
Before entering the field, chimpanzees displayed scanning behaviours (25 guarding postures, seven bipedal postures, two arboreal scanning in a high eucalyptus tree growing at the border of the field; clip S1) (Figure 4) and some individuals were not entering the field, staying at the edge. While in the maize field, chimpanzees were sometimes chased by barking dogs (N = 3 clips) or run after by the guardian of the field (N = 1), who threw a branch towards a severely mutilated adult female who hurried to cross the bridging tree. The screams and barks of chimpanzees during these events and the records of self-scratching behaviour and emission of soft/diarrheic faeces (Figure 4) in other occasions also indicated anxiety and perception of a risky situation. In twenty-two cases, chimpanzees came back from the field with ears (one to six pieces) or stems (six pieces) (clip S2) and among the half of them who were identified, six were females (carrying 14 items), five were males (bringing back nine items). Chimpanzees rarely transport wild food. Such transport of food items may indicate that chimpanzees were not ease to consuming crops in the field and felt at risk by staying in the field.

pone-0109925-g004: Frequency of signs of anxiety and vigilance in chimpanzees during day and night crop-raiding (occurrence of each behaviour per minute of video record).

Mentions:
Before entering the field, chimpanzees displayed scanning behaviours (25 guarding postures, seven bipedal postures, two arboreal scanning in a high eucalyptus tree growing at the border of the field; clip S1) (Figure 4) and some individuals were not entering the field, staying at the edge. While in the maize field, chimpanzees were sometimes chased by barking dogs (N = 3 clips) or run after by the guardian of the field (N = 1), who threw a branch towards a severely mutilated adult female who hurried to cross the bridging tree. The screams and barks of chimpanzees during these events and the records of self-scratching behaviour and emission of soft/diarrheic faeces (Figure 4) in other occasions also indicated anxiety and perception of a risky situation. In twenty-two cases, chimpanzees came back from the field with ears (one to six pieces) or stems (six pieces) (clip S2) and among the half of them who were identified, six were females (carrying 14 items), five were males (bringing back nine items). Chimpanzees rarely transport wild food. Such transport of food items may indicate that chimpanzees were not ease to consuming crops in the field and felt at risk by staying in the field.

Bottom Line:
More surprisingly chimpanzees were crop-raiding during the night.While nocturnal activities of chimpanzees have been reported during full moon periods, this is the first record of frequent and repeated nocturnal activities after twilight, in darkness.Habitat destruction may have promoted behavioural adjustments such as nocturnal exploitation of open croplands.

ABSTRACTIn a rapidly changing landscape highly impacted by anthropogenic activities, the great apes are facing new challenges to coexist with humans. For chimpanzee communities inhabiting encroached territories, not bordered by rival conspecifics but by human agricultural fields, such boundaries are risky areas. To investigate the hypothesis that they use specific strategies for incursions out of the forest into maize fields to prevent the risk of detection by humans guarding their field, we carried out video recordings of chimpanzees at the edge of the forest bordered by a maize plantation in Kibale National Park, Uganda. Contrary to our expectations, large parties are engaged in crop-raids, including vulnerable individuals such as females with clinging infants. More surprisingly chimpanzees were crop-raiding during the night. They also stayed longer in the maize field and presented few signs of vigilance and anxiety during these nocturnal crop-raids. While nocturnal activities of chimpanzees have been reported during full moon periods, this is the first record of frequent and repeated nocturnal activities after twilight, in darkness. Habitat destruction may have promoted behavioural adjustments such as nocturnal exploitation of open croplands.